1. Field of the Invention
The present invention relates to electronic circuits, and specifically to a method and apparatus for assessment of the integrity of a solder joint.
2. Background Art
In some electronic systems, components are coupled by xe2x80x9csolderingxe2x80x9d the components directly to a board. Soldering is the act of heating two metals and a solder alloy to form a xe2x80x9csolder joint.xe2x80x9d As the solder joint cools, an electrical and mechanical connection is formed. The reliability of the electronic systems depends in part on the integrity of its solder joints. Thus, it is desirable to characterize the integrity of solder joints so that solder joint behavior can be accurately predicted during manufacture and use.
The Soldering Process
The basic operational steps of soldering are as follows: (1) thorough cleaning of the metal to be joined by abrasive or chemical means; (2) application of a flux to remove oxides on heating and promote spreading and wetting of the solder; (3) alignment of parts to produce a controlled gap of 0.025 to 0.125 mm (0.001 to 0.005 inch); (4) application of heat; (5) feeding solder to the joint; (6) cooling without movement; and (7) removal of corrosive flux residues.
Solder is formed from a variety of metals. For example, tin, lead, silver, bismuth, indium, antimony and cadmium are frequently used in forming solder. Lead-tin alloy solders are widely used in the electrical industry. Solders are supplied in wire, bar, or premixed-paste form, depending on the application. Electronic circuits require a noncorrosive flux; fluxes based on rosin using alcohol as a carrier are sufficiently active to produce a good bond.
Soldering can be carried out using various heating methods, including, for example, conduction, infrared, vapor phase, hot gas, convection, induction, resistance and laser. Each method has its merits in cost, performance or operational convenience. For localized fast heating, laser offers the best performance followed by hot air. For uniform temperature, vapor phase is the best method. For versatility, volume and economy, convection and infrared are appropriate choices. Wave-soldering devices are prominent in printed-circuit production.
Solder Joint Failure
Solder joint failure often results when cracks form in the solder material because of a strain. In a lead-tin alloy solder joint, the solder joint deforms non-uniformly, with deformations forming primarily along the edges where the softest solder material is located. The lead-tin alloy in the deformations recrystallizes, and the recrystallized material is soft compared to the surrounding solder material. Further strain causes deformations near the recrystallized solder material, and the new deformations also recrystallize, thus extending the region of softer material. Continued strain results in cracks forming in the region of softer material. Ultimately, the cracks extend through the entire solder material, causing the solder joint to fail.
FIG. 1A illustrates a developing crack in a solder joint. At block 100, a strain is applied to a lead-tin alloy solder joint. At block 110, a region of recrystallized solder material appears. At block 120, the region of recrystallized solder material grows and a crack develops. At block 130, both the recrystallized solder material and the crack have spread across the entire solder joint, resulting in a solder joint failure.
Solder Joint Integrity Characterization
Electronic systems (e.g. general purpose computers) typically utilize a circuit board to couple electronic components of the system. A circuit board is comprised of circuitry printed on a firm planar surface. Soldered connections serve both to mechanically affix the components to the circuit board and to electronically couple components to the circuit board circuitry. Communications between these components travel through circuit networks contained on the circuit board. The soldered connections are made simultaneously on many electronic systems which are assembled on high-speed, automated production lines. The components of the electronic system are frequently soldered directly onto the circuit boards rather than to wire leads.
The reliability of solder joints is an important factor in the durability and design of electronic packages. However, advances in electronic technologies driven by the desire for miniaturization and increased circuit speed result in severe operating conditions for the solder joint and thus solder joint reliability problems. Specifically, the mismatched thermal expansion characteristics of the materials joined by the solder and the cyclic temperature fluctuations normally encountered during service constitute a condition of thermal fatigue for the constrained solder.
If the solder joints fail by physically breaking off the components, the entire electronic system may fail to perform properly. Thus, it is desirable to test and rate the mechanical strength of the solder joints. However, solder joint performance predicted by current methods of characterizing solder joint integrity fails to match actual solder joint performance. Prior art methods for characterizing solder joint integrity include shear tests, cold tensile tests and hot tensile tests. These methods are discussed in detail below.
FIG. 1B illustrates a shear test of a solderjoint. A constant rate of strain is applied to the solderjoint by a device from the side. The response force from the solder joint is measured using a force transducer. The response force is monitored until the solder joint fails. Prior art methods record the peak force generated in response to the continuously applied strain. This peak force is used to characterize the integrity of the solder joint and is meant to represent how well the solder joint withstands forces applied to it from the side.
FIG. 2 illustrates a cold pull test of a solder joint. A device grips the solder joint and applies a constant rate of strain to the solder joint, pulling it away from the circuit board. The response force from the solder joint is measured using a force transducer. The response force is monitored until the solder joint fails. Prior art methods record the peak force generated in response to the continuously applied strain. This peak force is used to characterize the integrity of the solder joint and is meant to represent how well the solder joint withstands forces which pull it away from the component surface.
FIG. 3 illustrates a hot pull test of a solder joint. A device is heated so that the solder joint partially melts and attaches to the device. Then, a constant rate of strain is applied to the solder joint, pulling it away from the circuit board. The response force from the solder joint is measured using a force transducer. The response force is monitored until the solder joint fails. Prior art methods record the peak force generated in response to the continuously applied strain. This peak force is used to characterize the integrity of the solder joint and is meant to represent how well the solder joint withstands forces which pull it away from the component surface. However, since the hot tensile test requires melting, and thus physically changing, part of the solder joint, reliability of this test is in doubt.
The failed solder joints which occur during circuit board assembly, testing, handling and field use do not have any relation to the results of the above testing methods because the mechanisms of failure do not directly correspond to simply applying a force either at the side of the solder joint or by pulling the joint away from the board. Rather, the mechanisms of failure for solder joints may comprise many types of destructive forces and each of these forces may affect the solder joint in a more or less destructive manner over time and as the force applied varies. The above characterization methods, therefore, are inadequate.
The present invention is a method for solder joint integrity assessment. The invention comprises collecting data from one or more solder joint strain tests and characterizing the solder joint integrity using a force-deflection graph to represent the collected data. A force-deflection graph has a force-deflection curve, which is the slope of a line that is formed on a graph which plots a solder joint""s response force to an applied strain versus time.
One embodiment of the invention uses the slope of the graph to characterize the integrity of the solder joint. Another embodiment uses the area below the slope of the force-deflection curve on the graph to characterize the integrity of the solder joint.
To generate the force-deflection graph, the invention first applies one or more tests to the solder joint. In one embodiment, a shear test is applied to the solder joint. In another embodiment, a cold pull test is applied to the solder joint. In another embodiment, a hot pull test is applied to the solder joint. In yet another embodiment, some or all of these tests are used to collectively obtain a more accurate characterization of the solder joint with which to generate the force-deflection graph. The present invention provides a characterization of solder joint integrity which more accurately predicts solder joint behavior than prior art methods.